- For over 100 years, National Parks have existed to preserve America’s natural and cultural heritage for current and future generations. As environmental pressures on wildlife and habitats have increased in recent decades, National Park lands have become important protected areas for many threatened and endangered species. Conservation and management of sensitive species requires a thorough understanding of population demographics, including population vital rates and the factors that affect them. The Northern Spotted Owl (Strix occidentalis caurina), a species designated as threatened under the Endangered Species Act of 1973 and Near Threatened by the International Union for the Conservation of Nature, occurs on public lands throughout the western United States including in Mount Rainier National Park. The demography of this population has been monitored for almost 20 years, and it represents the only demographic study area on federal lands on the west-side of the Washington Cascade Range. I used a multi-state, multi-season occupancy model in this study to obtain estimates of occupancy dynamics and breeding propensity for territorial Northern Spotted Owls at Mount Rainier National Park (MRNP), Washington USA from 1997-2016. I investigated relationships between occupancy dynamics, the presence of barred owls (Strix varia), habitat characteristics, and local and regional weather, at 36 territories occupied by spotted owls during the 20-year study period. I analyzed demographic data using a multi-state occupancy model, with three conditional occupancy states (unoccupied, occupied with no reproduction, occupied with reproduction), while allowing for imperfect detection. General occupancy of spotted owl territories in MRNP has declined by 50% in 20 years, although the decline has slowed in recent years (a pseudo-threshold trend). I also observed a positive relationship between occupancy dynamics and the mean slope of territories, with higher occupancy rates observed at territories with steeper terrain. Breeding propensity was negatively influenced by barred owls detected within historic territories, and positively influenced by early nesting temperatures across the study area. The probability that a site was occupied by breeding owls remained moderately high, but variable by year for territories previously occupied by breeding owls. However, overall occupancy of territories with breeding spotted owls decreased considerably over the course of the study reaching a low of 25% (95% CI:15-40%) in 2016. In addition, the ability to detect breeding spotted owls decreased when barred owls were present in the territory, which may be leading to underestimates of the breeding propensity of spotted owls. Habitat characteristics had no impact on spotted owl occupancy dynamics, which likely reflects the long history of habitat conservation in the park, with old forest predominating in most areas. My study illustrates the strong influence of barred owls on spotted owl demographics in a landscape where habitat loss by harvest or fire has not occurred. These results are concerning given that conservation policies of the park are focused on habitat preservation. Given the continuing range expansion of barred owls, habitat conservation may not be enough to ensure spotted owl persistence in the MNRP.